Question

The addition of hydrogen to unsaturated hydrocarbon is called as:
A) Hydrogenation reaction
B) Addition reaction
C) Both A and B
D) Substitution reaction

Answer 1

Hint: The unsaturated hydrocarbons like alkene and alkyne have a double or triple bond between the carbon atoms. These multiple bonds are reduced by the addition of hydrogen molecules. The hydrogen molecules when added to the unsaturated organic compounds form a saturated hydrocarbon.

Complete step by step answer:
The hydrogenation reaction is a reaction in which the hydrogen molecule $\text{ }{{\text{H}}_{\text{2}}}\text{ }$ is added to the carbon-carbon double bond or olefin .the hydrogen molecule is added to the organic compound and this is also called the reduction reaction. The hydrogen is added to the olefin. This is an additional reaction of hydrogen gas that takes place in presence of a catalyst. Let's consider an example of an ethane molecule. The ethane is the simplest olefin. The hydrogenation reaction of ethane involves the addition of hydrogen atoms across the double bond. Such that the double-bonded carbon atoms hold on an extra hydrogen atom followed by the breaking of one bond. The unsaturated alkene is converted into the saturated or single bonded hydrocarbon. The hydrogenation of ethane is as shown below,
$\text{ C}{{\text{H}}_{\text{2}}}\text{ = C}{{\text{H}}_{\text{2}}}\text{ + }{{\text{H}}_{\text{2}}}\text{ }\to \text{ C}{{\text{H}}_{\text{3}}}-\text{C}{{\text{H}}_{\text{3}}}\text{ }$
Like alkene, alkynes react with the hydrogen in presence of metal such as platinum $\text{ Pt }$ , palladium $\text{ Pd }$ , or nickel $\text{ Ni }$ to form an alkene. The general reaction of the alkyne is as shown below,
\[\text{ R}-\text{C}\equiv \text{CH + }{{\text{H}}_{\text{2}}}\text{ }\to \text{ R}-\text{C}{{\text{H}}_{2}}=\text{C}{{\text{H}}_{2}}\text{ }\]
This alkene further reacts with the dihydrogen and metal catalyst to give alkane. Let’s consider an example of propyne,
$\text{ C}{{\text{H}}_{\text{3}}}-\text{C}\equiv \text{CH + }{{\text{H}}_{\text{2}}}\text{ }\xrightarrow{\text{Pt/Pd/Ni}}\left[ \text{C}{{\text{H}}_{3}}-\text{CH=C}{{\text{H}}_{2}} \right]\xrightarrow{{{\text{H}}_{\text{2}}}}\text{ C}{{\text{H}}_{\text{3}}}-\text{C}{{\text{H}}_{\text{2}}}-\text{C}{{\text{H}}_{\text{3}}}$
Thus, the addition of hydrogen to an unsaturated hydrocarbon that is alkene or alkyne is called a hydrogenation and addition reaction.

Hence, (C) is the correct option.

Note: The addition of hydrogen molecules is also considered as the reduction reaction. As it removes the oxygen atom from the molecule and adds the hydrogen atom to the unsaturated compound.
The hydrogenation reaction of acid results in alcohol. The reduction of acid is as shown below,
$\text{ RCOOH + 2}{{\text{H}}_{\text{2}}}\text{ }\to \text{ RC}{{\text{H}}_{\text{2}}}\text{OH + }{{\text{H}}_{\text{2}}}\text{O }$
Note that, generally, the hydrogenation reactions are slow. The metal acts as a catalyst and provides the alternate pathway for the progress of the reaction by lowering the activation energy of the reaction.